3.10.1 Definition

Residual stresses are internal stresses between the composite constituents caused by the past history of the composite. Relevant history will include the fabrication or manufacturing sequence, heat treatments, thermomechanical working, changes in the moisture environment or annealing of radiation damage. Initially, stresses arise because the various constituents of the composite have different properties, in particular, stiffness and expansion coefficients. The residual stresses depend on the time-varying properties of the constituents, such as chemical reactions during curing and subsequent viscoelastic behavior of the matrix. The residual stresses found in as-fabricated composites are sometimes called curing stresses in the case of a polymeric matrix, although generally the curing process itself does not cause large residual stresses because of the soft or viscoelastic nature of the matrix at the fabrication temperature. The buildup of residual stresses is, however, related to polymer crystallization.

The uniform strain response to non-mechanical loads by the body as a whole leaves no resultant force or moment, provided the body is free to expand, bend, or twist. But since the diverse constituents are always bonded to each other, either mechanically or chemically, they are individually restricted in their usual dimensional response to the externally imposed loadings and/or environmental changes. As long as the constituents deform elastically and maintain interfacial bonding with each other, residual stresses will result. Poorer bonding, as in a porous composite, will allow greater individual accommodation to external changes, and the constituents will retain a lower residual stress state. These stresses,...